PROJECT SUMMARY Children with Down syndrome (DS) have an unusually high incidence of acute megakaryoblastic leukemia (DS-AMKL), a malignancy that results in increased numbers of abnormal megakaryoblasts, the platelet-producing cells in the blood. Studies have shown that mutations in GATA1, an essential hematopoietic transcription factor, are present in all cases of DS-AMKL as well as in the pre-leukemia of DS named transient myeloproliferative disorder (TMD). Nearly 20% of infants with TMD develop DS-AMKL within two years. Recent next-generation sequencing studies suggest that trisomy 21, the cause of DS, and a GATA1 mutation are sufficient for TMD, but that an additional genetic event is necessary for the transformation from TMD to DS- AMKL. Of note, more than half of patients with DS-AMKL have mutations in genes in the subunits of the cohesin complex, suggesting that a cohesin defect is a major driver of leukemic progression. The high incidence of cohesin mutations in DS-AMKL is in marked contrast to the 10% incidence in other hematologic malignancies, even in non-DS AMKL. Cohesin is a ring-shaped complex that regulates gene expression by forming chromatin loops between gene promoters and enhancer elements. Recent studies have shown that cohesin mutations lead to enhanced stem cell activity and myeloproliferative disease, and that they cooperate with FLT3-ITD to induce leukemia in vivo. How cohesin mutations contribute to DS-AMKL, however, has not been explored. My research aims to determine how trisomy 21 and GATA1 mutations cooperate with cohesin mutations to cause DS-AMKL. My specific aims are to: 1) Determine the contribution of cohesin mutations to leukemic transformation in vitro in a trisomy 21/ GATA1 mutant background; 2) Elucidate whether cohesin loss in the context of DS and a GATA1 mutation is sufficient to induce aberrant hematopoiesis and/or AMKL; and 3) Identify molecular targets of cohesin responsible for progression to leukemia. These studies will provide us with a better understanding of the mechanism behind DS-AMKL and likely lead to the identification of new therapeutic targets for this malignancy. My sponsor Dr. Crispino and I have jointly developed a powerful mentoring plan that, along with this F31 award, will enable my successful transition to a post-doctoral researcher and eventually to an independent principal investigator.